Thermionic tube



' Ju 1935- 0 F. E.- TERMAN 2,006,079 THERMIONIG TUBE Filed June 22, 1931' A I IN'VENTOR,

FREDERICK E. TERMAM ATTORNEY Patented June 25, 19 35 UNITED STATES PATENT OFFIC My invention relates to thermionic discharge devices or vacuum tubes, and its broad purpose is to provide a tube of low resistance, and with a minimum of power expended in heating the cathode thereof. Objects of this invention are to reduce the space charge adjacent the filamentary cathode of the tube, and to provide a reservoir of electrons such that the effective area of the cathode is much greater than its actual geometrical area.

Other objects of this invention will be apparent or will be specifically pointed out in the description forming a part of this specification. but I do .not limit myself to the embodiment of my invention herein described, as various forms may be adopted within the scope of the claims.

Referring to the drawing:

Figure 1 is a longitudinal section of a triode, illustrative of my invention.

' Figure 2 is a transverse sectional view of the de vice illustrated in Figure 1, the plane of section being on the line 2--2 of Figure l.

Figure3 is a diagrammatic plan view, showing a modified electrode arrangement.

The impedance of a vacuum tube of the thermionic type is a function of the areas of its cathode and anode. Ordinary tube structure provides a large area for the anode, which makes the cathode area usually the limiting factor in the impedance.

0 This cathode area is usually relatively small, partly owing to the fact that the power available for heating the cathode is necessarily small, partly to customary filamentary form, and partly to position within the anode. Under operating conditions the cathode emits electronsat a rate which is determined by the potential gradient adjacent its surface. The cloud of emitted electrons, each carrying a negative charge, reduces this poten tial gradient, and thus limits the rate at which electrons are emitted-the so-called space charge effect. If the electrons can be rapidly ,withdrawn from adjacent the cathode surface, and re-distributed in a storage. area of larger dimensions than the cathode, the effect is the same as though a'larger cathode were used. More electrons will be drawn from cathode to anode for a given potential difference, and the impedance of the tube will be materially decreased. My invention accomplishes this end.

, Broadly considered. my invention comprises the use of a cathode which is disposed as a cage, preferably of substantially cylindrical form, to

, provide a space separated from the anode by the cathode. Electrons emitted from the cathode fill this space, which therefore acts as a storage area, from which electrons can be drawn between the cathode elements to carry current to the anode. An electrode mounted within this space assists in distributing the electrons emitted from the filament, and thus further decreases the im- 5 pedance. This electrode should preferably be at a potential different from that of the cathode.

If this electrode be negative, it will accomplish this re-distribution .to a certainextent. The

electrode may therefore be unconnected with any 10 of the tube elements, either within or Without the tube, under which circumstances it will acquire a negative charge from the electron cloud. The best results are obtained, however, by making the distributing electrode in filamentary form, 1 and giving it a positive charge. The effect of this isthat the distributing electrode draws the electrons rapidly away from the cathode toward itself, thus reducing the space charge near the cathode elements, but very few of the electrons 20 thus drawn actually reach the electrode owing to its small area, continuing on past it and into the spaces between the cathode elementson the opposite side of the cage. As a result of this action the area of the cathode becomes effectively the entire area of the surface outlined by the cathode cage. instead of the very small portion of this surface which is actually occupied by the electron emitting surface.

It will be obvious that many detailed structures may be utilized to give the effect described. The particular form chosen for illustration comprises the usual glass envelope I, into the base of which is sealed a stem 2. A pair of support wires 3 are held in the SIElll, and to these wires are welded horizontal rings 4 and 5.

These rings serve to position insulating disks r and 8, which may be of mica, isolantite or other refractory insulator. and a filament wire ill is Woven back and forth through holes H in the disk, to define a substantially cylindrical cage, of which the two disks form the top and bottom, held against the support rings by the filament wire.

The drawing shows he two sides of the filament as connected to leads l2, sealed through the stem, so that the two halves of the filament are in parallel. It is, of course, possible to connect all of the filament wires together at the top, and again at the bottom. so that all are in parallel, or to adopt any other parallel or series-parallel arrangement between the wires which will give any required voltage-current characteristic to the filament. Another possible arrangement, would be to make the filament helical so that it would approximate the ordinary vacuum tube grid in form. The common feature in all of these arrangements is that the filament forms a cage which encloses an open central space.

Surrounding the cathode is the usual grid, com prising in the present case the helical coil i3 welded to the support wires l5 and IS, the latter of which extends through the stem 2 to give an external connection to the grid.

The anode or plate I! surrounds the grid, being welded to the lead l8 and the supporting dummy 20.

In the form of the tube here shown, the distributing electrode is connected directly to the anode. A bridge wire 2! extends across the top of the anode, and to it is fastened an extremely fine filamentary electrode 22 which passes, as nearly axially as possible, down through the center of the cathode cage and connects to an anchor 23, held by the stem.

As stated above, it is unnecessary that the electrode 22 be actually connected to the anode as shown. Its function will be partly, though less completely, accomplished. even if it be isolated from other parts of the tube circuit. Connecting the distributing electrode as shown has the obvious .disadvantage that it will draw some current, but by making it of extremely fine wire the magnitude of this current becomes negligible, and direct connection to the anode, the highest positivepotential in the circuit, allows electrode 22 toexercise its distributing function with greatest effectiveness.

'Iube designs for difierent purposes are so various that it is obviously impossible to show the many ways in which the fundamental idea of providing an electron storage space may be embodied. Any space may be used for this purpose which is separated from the anode by the surface within which the cathode lies. One modification is shown diagrammatically in Figure 3, wherein the cylindrical anode 25 is surrounded in turn by the concentric grid 26 and cathode cage 21; The distributing electrode 28, in this case preferably operated at negative potential, may surround the filament as shown, or the electrode may be omitted and its function assumed by the negative charge accumulating on the wall of the glass envelope 38 from the filament. Other possiblearrangements will occur to those skilled in the .art.

I claim:

1. A thermionic discharge device comprising an anode, a filamentary cathode disposed to enclose acentral space. and a filamentary electrode mounted within the space and connected to said anode.

2. A thermionic discharge device comprising an anode, a filamentary electron emitting cathode disposed as a substantially cylindrical cage surrounding a central space, and a filamentary electrode mounted substantially axially within'said space and connected within the device to a point normally positive to said cathode.

3. A thermionic discharge device comprising an anode, a filamentary cathode disposed as a substantially cylindrical cage surrounding a central space. and'a filamentary electrode mounted substantially axially within said space and connected to said anode.

4. A thermionic discharge device comprising a filamentary electron emitting cathode, an anode, a control electrode mounted between said cathode and anode. and a filamentary electrode for dis tributing the electrons emitted from the cathode and mounted on the opposite side of the filament from the anode and exposed to the emission of said cathode.

FREDERICK E. 'IERMAN. 

